This invention relates to the production of uranium and more particularly to the recovery of uranium from saline lixiviants by ion exchange adsorption.
Uranium is produced from uranium-bearing ores by various procedures which employ an alkaline or acid lixiviant to leach the uranium from its accompanying gangue material. Typically, the alkaline lixiviants contain alkali metal or ammonium carbonates or bicarbonates or mixtures thereof which function to solubilize hexavalent uranium in the ore as anionic uranyl complexes such as uranyl tricarbonate ions. The pH of the lixiviant normally is within the range of about 8 to 10. The acid lixiviants usually are formulated with sulfuric acid which solubilizes hexavalent uranium as complex uranyl sulfate anions. The sulfuric acid normally is used in a concentration to maintain a pH between about 0.5 to 2.0.
The leaching operation may be carried out in conjunction with surface milling operations in which uranium ore obtained by mining is crushed and blended prior to leaching, heap leaching of ore piles at the surface of the earth, or in-situ leaching in which the lixiviant is introduced into a subterranean ore deposit and then withdrawn to the surface. Regardless of the leaching operation employed, the pregnant lixiviant is then treated in order to recover the uranium therefrom. One conventional uranium recovery process involves passing the pregnant lixiviant over an anionic ion exchange resin and then eluting the resin with a suitable eluant to desorb the uranium from the resin. The resulting eluate is then treated to precipitate uranium therefrom to produce the familiar "yellowcake."
The anion exchange resins employed for uranium concentration are characterized by fixed cationic adsorption sites in which the mobile anion, typically chloride or another halide, is exchanged by the uranyl complex anion. Such anionic ion exchange resins are disclosed, for example, in Merritt, R. C., THE EXTRACTIVE METALLURGY OF URANIUM, Colorado School of Mines Research Institute, 1971, pp. 138-147. Suitable anionic ion exchange resins may take the form of polymers or copolymers of styrene substituted with quaternary ammonium groups or polymers or copolymers of pyridine which are quaternized to form pyridinium groups.
The adsorption of uranium from acid and carbonate solutions is described by Merritt at pages 147-156 where it is recognized that the presence of inorganic salts in the pregnant lixiviant tends to reduce adsorption of uranium by the anion exchange resin. Thus, Merritt discloses at pages 147, 148, and 152 that high chloride ion concentrations tend to reduce the adsorption of uranium by the resin and thus result in decreased resin loading.